Because a meter can read a scene (placing it in Zone 5, or 18% gray) does not mean that's how it would be reproduced. Assuming that the sky at the zenith would be properly recorded in Zone 7 or 8 and shodow areas in Zone 2 or 3 then we're really not dealing with much more than 10 stops of "real image" brightness levels. Sure there are things that are brighter than a sunny day at the beach, and dimmer than a 30 watt bulb on a newspaper, but typically not in real world images.

If you read EV 18 in the highlights and EV 8 in the shadows AND you want them both to appear 18% gray, then yes, there is a 10 stop range. But, if you want bright areas to be bright and dark areas to be dark then this isn't the case.

I believe that Michael's informative article on histograms inadvertently short-changes one of photography's most difficult challenges, namely exposure latitude. Michael states that the range of brightnesses is about 10 stops, but in fact it's at least twenty.

My Pentax digital spotmeter shows "EV" numbers, each increment of which indicates a change of one stop. EV 8 is twice as much "brightness" as EV 7. The scale on the meter runs from 1 to 20.

I know from experience that reading the brightness of the brightest objects in nature (the reflection of sunlight on water or ice) results in readings over EV 19. Blue sky at the zenith usually reads very close to EV 13. Similarly, the meter will accurately measure brightnesses down to EV 1 or EV 2.

Twenty stops is 2 to the twentieth power, a very large number. If my math is correct, it's four million, meaning that the brightest objects we can expect photograph are at least four million times brighter than the darkest.

Not that we would expect a single scene to exhibit the entire range of 20 stops, of course, but it in my experience, natural scenes can exceed 10 stops of brightness range.

Not trying to start an argument, but I believe many "real world" scenes exceed 10 stops - typically when reflections of the sun on water are involved.

I often read EV 17 or 18 on sunlit clouds, 19 or 20 on water reflections, yet see EV 8 or less in shadows, all in the same scene. Sunny day photography in dense coniferous forest presents this problem constantly.